A stroke-enlarging type brake booster mentioned herein means one wherein a power piston, which is so disposed in a booster casing as to divide the interior of the same into two chambers and actuated by a pressure difference between the two chambers, is separated from a controlling mechanism for controlling the pressure difference upon operation of an input member for being relatively movable to the controlling mechanism. This stroke-enlarging type brake booster is featured in obtaining a larger output stroke than the input stroke, while in ordinary brake boosters the output stroke can never be larger than the input stroke.
The stroke-enlarging type brake booster is still disadvantageous, when there is a certain limit in an assisting or promoting force, i.e., a force produced by the difference of pressure on opposite sides of the power piston, in that an attempt of getting a larger output than a critical output obtained at the upper limit of the assisting force results in a temporary rapid increasing of the input stroke without a resultant increase of the braking force.
A stroke-enlarging type brake booster includes a power piston disposed in a casing to divide a space within the casing into two chambers and moved in accordance with the difference of pressures in the chambers on opposite sides thereof, a control piston which has a control valve for controlling the pressure difference in the two chambers and a transmission mechanism for transmitting an input force of an input member to an output member and which is axially and relatively movably fitted in the power piston, a reaction lever for transmitting a force from the power piston and the transmission mechanism to the output member, and a return spring for biasing the power piston in backward or retracting direction. The reaction lever abuts at an output portion in the middle thereof on the output member and further abuts at one of a pair of input portions located on opposite sides of the output portion, on the transmission mechanism and at the other input portion on the power piston.
In a brake booster of this type an operating stroke of the input member and the transmission mechanism at the initial stage of braking operation is small, but a larger stroke is obtained in the output member than in the input member due to a pivotal movement of the reaction lever which is caused by a stroke of the power piston advanced by an assisting or promoting force generated by the pressure difference. When however the assisting force to the power piston reaches, after the power piston has advanced a certain predetermined amount relative to the control piston, its limit or critical point the power piston begins to be pushed back due to a reactionary force from a master cylinder delivered by way of the reaction lever, which produces the above-mentioned rapid input stroke increasing phenomenon.
The applicants of this invention are two of three co-inventors of a brake booster which was developed with an object of eliminating the above disadvantage. This brake booster has an anti-backdown transmission mechanism which is provided with a ball interposed in a bridging manner between two recesses respectively formed in the internal peripheral surface of the power piston and in the external peripheral surface of the control piston, and a ball retainer for loosely retaining the ball and abutting on the reaction lever. This development is shown in the copending patent application Ser. No. 194,404, filed Oct. 6, 1980.
In this anti-backdown transmission mechanism the reactionary force of the reaction lever is little applied, after the assisting force to the power piston has reached the limit, to the power piston, but mainly applied to the control piston, which effectively prevents the occurrence of the rapid input stroke increasing phenomenon. Even this mechanism could not be perfect in respect of returning movement of the ball retainer to the original position, which was made clear by further study of the applicants of this invention.
The problem resides in that the force causing to return the ball retainer to the original position is the reactionary force of the reaction lever, while the force causing to return the power piston to the original position is the elasticity of the return spring. The reactionary force is fairly large only while the braking is in effect, that is while the power piston and the control piston are considerably away from their original position, but it is inevitably diminished in the final stage of the returning stroke of the power piston. This sort of reactionary force is unreliable and unsuitable as the returning force for the ball retainer. Besides, this way of returning of the ball retainer may possibly cause a time delaying in the returning. The imperfect and delayed returning of the ball retainer to the original position hinders the normal returning of the output member and possibly causes an unexpected positioning thereof in relation to the power piston, which contains a risk of a removal of the ball out of the ball retainer.